EP0238972A2

EP0238972A2 - Call meter signal receiver for subscriber's private metering of telephone system

Info

Publication number: EP0238972A2
Application number: EP87103833A
Authority: EP
Inventors: Kazuhiro Sato; Yoshimi Iijima; Hiroyoshi Mori
Original assignee: Fujitsu Ltd
Current assignee: Fujitsu Ltd
Priority date: 1986-03-20
Filing date: 1987-03-17
Publication date: 1987-09-30
Anticipated expiration: 2007-03-17
Also published as: DE3767898D1; US4750200A; EP0238972A3; CA1259432A; JPH0473827B2; AU574603B2; AU7020087A; EP0238972B1; JPS62220054A

Abstract

A call meter signal receiver for subscriber's private metering (SPM) system operated by a call meter signal sent from a central office (CO). The receiver is operable both by common mode and normal mode signals. The receiver is so small in size that it can be installed in a central office trunk (COT) of the Private Branch Exchange (PBX). The call meter signal receiver composed of a frequency tuned receiver (44) comprising two buffer circuits (41, 42), the first one (41) for receiving the common mode and the second one (42) for receiving the normal mode signal. The first buffer circuit (41) is connected to the input side of a hybrid circuit (23) of the COT and has a high impedance, while the second buffer circuit (42) is connected to one of the output terminals of the hybrid circuit (23) for receiving the normal mode signal and has high impedance for voice signals. The outputs of these buffer circuits (41, 42) are fed to the frequency tuned receiver (44) composed of a phase locked loop (PLL) of which frequency is tuned to 50 Hz, 12 kHz or 16 kHz by switching externally attached circuit composed of capacitor and resistors. The call meter signal receiver is so small sized that it can be installed in a central office trunk (COT) provided in a private branch exchange (PBX) equipment.

Description

The present invention relates to an equipment installed in trunk circuits of a private branch exchange (PBX) system for receiving call meter signals, more precisely it relates to small sized subscriber's private metering (SPM) equipment which are operable by various types of metering pulses having common mode or normal mode.
There are various kinds of charging equipment installed in telephone offices for counting calls of each subscribers for charging tariff. But recently the demand for private metering is increasing, especially among PBX subscribers or modern push button dialing telephone subscribers. Such equipment is called, subscriber's private metering and abbreviated as SPM. Most of such SPM systems are controlled by their own clock and counter to operate their call meters. But some of the PBXs are provided with SPM equipment which are operated by metering signal sent from the telephone office, and the demand for such SPM system is increasing.
There are various kins of metering signals used in telephone offices, but they can be classified into 50 Hz, 12 kHz and 16 kHz signals. And there are two modes of metering signals called as common mode and normal mode. The common mode signal is transmitted in each of the two-wire telephone line with common phase (same phase) to each other, so it is sometimes called as parallel mode, and is mainly used for low frequency metering signal of 50 Hz. While the normal mode signal is transmitted in the two-wire line with opposite phase to each other. So, it is sometimes called a differential mode signal, and is mainly used for high frequency metering signals of 12 kHz or 16 kHz.
The SPM equipments which are operated by such metering signal sent from the telephone office have to change its circuit according to the type of the signal sent from the telephone office. The matching of the SPM equipment to the metering signal is done by replacing a printed circuit board or by switching to a proper circuit among various ones which are 'installed in advance in the equipments corresponding to expected types of the metering signals.
In order to make clear the advantage of the present invention, prior art SPM equipment will be described briefly. Fig. 1 is a block diagram of a prior art PBX system having a SPM function. A PBX cabinet 1 is installed in a subscribers office, and performs as an extended branch service between a central office (CO) 9 and branch telephones 3. Though there is shown only one telephone and one central office line 10 in the figure, usually a plurality of central office lines and telephones are interconnected and exchanged by a time division switch (TDSW) 2. Each of the telephones 3 is connected to the TDSW 2 via a line circuit (LN) 4 which is an interface circuit between the telephone and the time division switch 2. On the central office side of the TDSW is provided a central office trunk (CO trunk or COT) 5 which is and interface circuit between the central office line and the TDSW. These equipments are controlled by a common control unit (CC) 6. The PBX cabinet 1 is provided with an attendant console (ATT) 7, which commands the operation of the PBX manually by an operator. There may be various equipment provided in the PBX, but only the parts relevant to the present invention are shown.

I

Between the central office (CO) 9 and the PBX cabinet 1 is provided a SPM cabinet 8. The telephone signal and the call meter signal (sometimes it is abbreviated as meter signal) are sent from the central office to the PBX via a telephone line 10 which is usually a two-wire line. These signals are separated from each other in the SPM cabinet 8 and transferred to the PBX cabinet 1 respectively through a speech line 11 and a meter line 12. In the figure, the path of the call meter signal is identified by a broken line. The call meter signal is treated by the common controller 6 and operates an indicator. The indicator may be a call meter (MET) 13 or any other display means such as a printer, or a cathode ray tube etc.
Fig. 2 shows an example of a prior art SPM receiver for a common mode call meter signal. The equipment of Fig. 2 correspond to the SPM cabinet 8 and COT 5 in Fig. 1. Throughout the drawings hereinafter, same reference numerals or symbols designate same or similar parts. The meter signal is sent from the central office together with a voice signal via a two-wire telephone lines Ll and L2. The meter singal is composed of a pulse burst of 50 Hz. having same phase to each other in both of the lines Ll and L2, while the voice signal is a normal mode signal which has opposite phase to each other in theses lines. These two signals are fed to the central office trunk (COT) 5 of the PBX system.
On the central office side of the COT 5, there is provided a 50 Hz receiver 21 between the two-wire line Ll, L2 and the ground, which is designed to be high impedance for voice signal frequency but it becomes very low impedance for 50 Hz. So, the voice signal is fed to the COT without loss, but the call meter signal of 50 Hz is shunted to the ground and does not appear in the COT 5. In Fig. 2, broken lines show the path of the call meter signal, and arrow marks show their phase, indicating that they have equal phase to each other. Even if a fractional part of the 50 Hz signal appeared in the COT 5, it can not go further to the right in the figure. Because the COT 5 is provided with a hybrid circuit 23 which does not convert the two-wire common mode signal to a 4-wire signal, while the voice signal is converted to the 4-wire signal because it has a normal mode. In the figure, a symbol BN designates a balancing network for the hybrid circuit 23.
The output of the hybrid circuit 23 is fed to the TDSW 2, and switched to the destination telephone equipment 3 (not shown). Above is the flow of signal from the central office to the destination telephone equipment. The flow of signal from the telephone equipment 3 to the central office is also transmitted through the same path but in a reverse direction as described above. A reference numeral 24 designates an interface circuit which terminates the two-wire lines L1 and L2, and provides various functions such as dial pulsing, supervising, ringing and so on. These functions are controlled by a controller 25, but such operations are the same in all ordinary PBX systems, and they are not explicitly relevant to the present invention, so, further description is omitted for the sake of simplicity.
When the call meter signal is sent from the central office, the signal runs through the 50 Hz receiver 21 to the ground, so the call meter signal is detected by the 50 Hz receiver 21. Then, the 50 Hz receiver 21 closes a switch 26 to make a loop between the controller 25 and the lines MT, MR. This closed loop is detected by the controller 25 and a signal indicating that a meter signal is received is sent to the common controller (CC) 6.
Fig. 3 is an example of a prior art SPM receiver for a call meter signal of normal mode. In the figure, broken line indicates the flow of the call meter signal, and arrow marks indicates that the phase of the meter signal is in opposite phase to each other in the lines L1 and L2. Such circuit is used for call meter signals of 12 kHz or 16 kHz. On the lines Ll and L2 is provided a band rejection filter 31 which is tuned sharply to 12 kHz or 16 kHz corresponding to the frequency of the call meter signal. So, the voice signal (400 - 3,600 Hz) is transmitted to the interface circuit 24 without loss. While the call meter signal is branched to a 12 kHz/16 kHz receiver 32 by a transformer T 33. The circuit is designed that input impedance of the transformer 33 be very high for voice frequency, but it is designed to become low impedance for 12 kHz/16 kHz. So, the call meter signal sent from the central office is detected by the 12 kHz/16 kHz receiver 32. The -operation of the remaining parts of the PBX is same as that of the circuit of Fig. 2.
More of such prior art SPM equipments are disclosed in, for example,
Japanese Laid Open Patent 58-19071 (published on Feb. 3, 1983) by K. Sato, or
Japanese Laid Open Patent 58-159054 (published on Sep. 21, 1983) by K. Sato
As has been described above, the prior art SPM equipments have to change their circuits corresponding to the call meter signals sent from the central office. The matching of the circuit to the meter signal is done by changing a printed circuit board. Some of the universal SPM equipments are installed with various circuit boards corresponding to expected type of the meter singal to deal with, and the circuit is switched manually to a proper one according to the type of signal sent from the central office. Therefore, the SPM cabinet becomes large, and expensive.
It is a specific object of the invention to provide a call meter signal receiver which is operable by both common mode and normal mode signal.
It is a further object of the present invention, therefore, to provide a small sized and cheap universal SPM equipment which can be operated by various types of the call meter signal.
The call meter signal receiver of the present invention is composed of a frequency tuned receiver having two input buffer circuits, the first buffer circuit receives the common mode meter signal, and the second buffer circuit receives the normal mode meter signal. The input terminal of the first buffer circuit is connected in parallel to both of the input terminals of the hybrid circuit of the COT, so it picks up the common phase signal but it does not affect the normal mode signal. The input terminal of the second buffer circuit is connected to one of the output terminals of the hybrid circuit, so it picks up only the normal mode signal. Therefore, the transformers or band rejection filters which takes a large space are all eliminated.
The outputs of these buffer circuits are fed to the frequency tuned amplifier. The receiving frequency of the frequency tuned amplifier is sharply tuned to 50 Hz, 12 kHz or 16 kHz, the frequency of the call meter signals, by a phase locked loop (PLL) circuit. The tuning frequency of the PLL circuit is variable by externally attached circuit composed of a capacitor and resistors. Therefore, the receiving frequency may be switched to any one of the above call meter signals by switching the externally attached circuit.
These circuits are miniaturized using integrated circuit (IC), so they can be put into the central office trunk circuit (COT) of the PBX cabinet without affecting the prior art circuit of COT. Accordingly, the SPM cabinet is eliminated, and hence the system size and its cost are reduced.

Fig. 1 is a block diagram of a prior art PBX system having a SPM function.
Fig. 2 is a block diagram of an exemplary prior art SPM meter signal receiver for a common mode call meter signal.
Fig. 3 is a block diagram of exemplary prior art SPM meter signal receiver for a call meter singal of normal mode.
Fig. 4 is a block diagram to show a general configuration of the SPM system of the present invention.
Fig. 5 is a a circuit diagram of a call signal receiver for SPM system embodying the present invention.
Fig. 6 is a block diagram showing how the SPM system is simplified by applying the present invention.

Throughout the drawings, same reference numerals or notations designate same or similar parts.
The invention will be described referring to a preferred embodiment. Fig. 4 is a block diagram illustrating a general configuration of the SPM system of the present invention. Fig. 4 shows a control office trunk circuit (COT) 5 of a PBX station embodying the present invention. The central office lines L1 and L2 (usually they compose a two-wire line) are connected directly to the interface circuit 24. The output of the interface circuit 24 is connected to the hybrid circuit 23 which converts the 2-wire signal to a 4-wire signal. These inter face circuit and the hybrid circuit 23 are similar to those of prior art.
The input terminals of the hybrid circuit 23 are shunted by resistors R6 and R7 having a same high resistance value. From the junction point of R6 and R7 is taken out a common mode signal and fed to the frequency tuned receiver 44 via the first buffer circuit 41. The frequency tuned receiver 44 is tuned sharply to 50 Hz by the phase locked loop (PLL), and it has a comparative input impedance to the resistance value of R6 and R7. So the common mode meter signal is detected by the frequency tuned receiver 44. It will be apparent that the normal mode signals are not affected by these circuits. On the contrary, the common mode signals cannot be transferred to the output side of the hybrid circuit 23.
From one output terminal 43 of the hybrid circuit 23 is taken out the normal mode signal, and is fed to the frequency tuned receiver 44 via the second buffer circuit 42. The frequency tuned receiver 44 is sharply tuned to 12 kHz or 16 kHz by a PLL circuit. So, the normal mode meter signal is detected by the frequency tuned receiver 44. But the voice signals are not affected by these circuits, because the second buffer circuit 42 has a high input impedance for voice signal. The normal mode call meter signal can not be transmitted to the right in the figure, because ordinary voice signal switching equipments which are connected to the right in the figure are cut off from the high frequency of 12 kHz/16 kHz.
The outputs of the frequency tuned receiver 44 is sent to the common controller (CC) 6, and the SPM equipments are controlled. These controls are similar to those of prior art SPM systems.
Fig. 5 shows a circuit diagram of a meter signal receiver embodying the present invention. The circuit is composed of a frequency tuned receiver 44 and two buffer circuits 41 and 42 which are used respectively for the 50 Hz and 12 kHz or 16 kHz signals. The circuit is miniaturized using ICs. These ICs are all conventional ones available on the market. The input buffer circuit 41 for 50 Hz, and the input buffer circuit 42 for 12 kHZ/16 kHz comprise respectively an operational amplifier 51 and 52. These operational amplifiers 51 and 52 are integrated in a single IC package, for example MB 3615 fabricated by Fujitsu. The frequency tuned receiver 44 is available as an IC package, for example XR-2211 fabricated by Integrated Systems Inc. (Sunnyvale, Cal. 94088).
Referring to Fig. 5, the common mode meter signal is taken out from a junction point of the resistors R6 and R7 which are connected respectively to opposite input terminals of the hybrid circuit 23. The value of resistance of R6 and R7 is 100 k Ohms for example. The circuit constants disclosed hereinafter are all typical values, therefore, several modifications are possible. The common mode signal is fed to the non inverting input side (which is denoted as + side herein after) terminal of the first operational amplifier 51 via a capacitor C1 of 0.1 _/u F, and a resistor Rll of 500 k Ohms connected in series. Therefore, the input impedance of the 50 Hz buffer circuit 41 becomes very high. The + terminal of the first operational amplifier 51 is grounded through a resistor R5 of 12 k Ohms. The output of the first operational amplifier 51 is fed back to the inverting side input terminal (which is denoted as - side hereinafter) of the first operational amplifier 51. The output signal of the first operational amplifier 51 is fed to paired diodes Dll and D12. These diodes are connected in antiparallel to each other, and perform as a noise limiter which eliminates the low voltage signals less than the forward junction voltage of the diodes. The pulse burst of the normal mode meter signal is then fed to a frequency tuned receiver 44 via a capacitor C3 of 1.5 µF.
The normal mode meter signal is taken out from.one output terminal 43 of the hybrid circuit 23, and is fed to the second buffer circuit 42 via a series connected capacitor C2 and a resistor Rl. Capacity of C2 is -1500 pF, and the resistance of R1 is 10 k Ohms. So, the input impedance of the second buffer circuit 42 becomes high for voice signal frequencies. The meter signal is fed to the - terminal of the second operational amplifier 52. The + terminal of the second operational amplifier 52 is grounded. The output of the second operational amplifier 52 is fed back to its - input terminal via a resistor R2 of 300 k Ohms. The output signal of the second operational amplifier 52 is led to the diode pair Dll and D12 via a resistor R3 of 1 k Ohms, and then fed to a frequency tuned receiver 44.
The common mode and normal mode meter signals are thus fed to the frequency tuned receiver 44. The frequency tuned receiver is available as an IC package. In the embodiment, an IC of RX-2211 has been used. It is provided with a PLL circuit which is tunable to desired frequency by varying externally attached capacitor and resistor. When the frequency of the input pulse burst has equal repetition rate to the tuned frequency of the PLL circuit, the frequency tuned receiver 44 outputs a logic signal, a meter pulse MP, of approximately 5 volts which is sufficient to drive ordinary common control circuit of the SPM system. In an embodiment, the external capacitor C_o was 0.01 _/u F, and the resistances of R8, R9 and R10 were respectively 2 M Ohms, 8.06 k Ohms and 6.04 k Ohms for 50 Hz, 12 kHz and 16 kHz respectively. The resistors R8, R9 and R10 are switched manually by a switch 54, so the receiving frequency of the receiver is sharply tuned to 50 Hz, 12 kHz or 16 kHz which is equal to the frequency of the meter signals expected to be sent from the central office. In practice, only one frequency and one mode of these meter signals is used in one central office, so the SPM receiver is switched to corresponding frequency when the equipment is installed in the PBX by using the switch 54.
As mentioned before, since the circuit of Fig. 5 is so small that it can be installed in the CO trunk of the PBX without disturbing the prior art CO trunks. The general configuration of the SPM system embodying the present invention, therefore, becomes as shown in Fig. 6. Comparing it to the prior art system of Fig. 1, the SPM cabinet has been eliminated and the CO lines are directly connected to respective COT 5. So, the system is simplified and the economic effect will be apparent. In the embodiment of Fig. 6, a printer 13' is used for displaying the output of the call meters. But it can be replaced by or commonly used with any of display unit such as a cathode ray tube, a count meter and so on.
In above disclosure, two buffer circuits (51 and 52) have been used. But various modifications are possible, for example, the number of the buffer circuit and the frequency tuned receiver may be increased depending on the number of types of the meter signals to deal with. And some of them are connected to the input side of the hybrid circuit for receiving the common mode, and the remainders are connected to the output side of the hybrid circuit for receiving the normal mode signal. These modifications are all in the scope of the invention.
As has been described above, by applying the present invention, the SPM system can be operated by a call meter signal from a central office, and the SPM system of PBX can be operated by any type of the call meter signals sent from the central office. Therefore, there is no need to change the circuit board to meet the type of the signal. And the call meter signal receiver is made small size, so it can be installed in a CO trunk of the PBX terminal, so, a large expensive filter circuits, transformers, cabinet and so on which occupies a large space in the SPM system can be eliminated. Therefore, a very economic small sized SPM system is realized.

Claims

1. A call meter signal receiver for subscriber's private metering (SPM) system which is installed in a central office trunk (COT) provided in a private branch exchange (PBX) equipment and is operated by a call meter signal sent from a central office (CO), said call meter signal receiver comprising:

plurality of buffer circuits (41, 42) for respectively receiving a predetermined type of call meter signals sent from a central office (CO); and

a frequency tuned receiver (44) for detecting said call meter signal, said frequency tuned receiver (44) being tuned to the frequency of said call meter signals.

2. A call meter signal receiver as set forth in claim 1, further comprising a switching means for switching the tuning frequency of said frequency tuned receiver (44) to one of predetermined frequency of said call meter signals.

3. A call meter signal receiver as set forth in claim 1, wherein said frequency tuned receiver comprises a phase locked loop (PLL) circuit for stabilizing the receiving frequency, and said receiving frequency is tuned to the frequency of call meter signal by choosing an externally attached circuit composed of a capacitor and resistors.

4. A call meter signal receiver as set forth in claim 1, wherein said buffer circuits (41, 42) are composed of:

a first buffer circuit (41) for receiving a common mode signal;

and a second buffer circuit (42) for receiving a normal mode signal.

5. A call meter signal receiver as set forth in claim 4, wherein said COT comprises a hybrid circuit (23) for converting 2-wire signal to 4-wire signal, said first buffer circuit (41) is connected to an input side of said hybrid circuit (23), and said second buffer circuit (42) is connected to an out side of said hybrid circuit (23), and both outputs of said buffer circuits (41, 42) are fed to said frequency tuned receiver (44).

6. A call meter signal receiver as set forth in claim 5, wherein said first buffer circuit (41) being connected to a junction point of two resistors (R6, R7), and the other ends of said resistors (R6, R7) are respectively connected to opposite input terminals of said hybrid circuit (23), and said second buffer circuit (42) is connected to one of the output terminals of said hybrid circuit (23).

7. A call meter signal receiver as set forth in claim 5, wherein said first buffer circuit (41) has a high input impedance, and said second buffer circuit (42) has a high input impedance for voice signals.

8. A call meter signal receiver for subscriber's private metering (SPM) system which is installed in a central office trunk (COT) provided in a private branch exchange (PBX) equipment and is operated by a call meter signal sent from a central office (CO), said call meter signal receiver comprising:

a first frequency tuned receiver for receiving common mode call meter signal, and being connected to a junction point of two resistors (R6, R7) of which other ends being respectively connected to opposite terminals of the two input terminals of a hybrid circuit (23) provided in said COT; and

a second frequency tuned receiver for receiving normal mode call meter signal, said second frequency tuned meter being connected to one of output terminals (43) of said hybrid circuit (23).

9. A call meter signal receiver as set forth in claim 8, wherein said first frequency tuned receiver is tuned to the frequency of common mode call meter signal, and said second frequency tuned receiver is tuned to the frequency of a normal mode call meter signal.

10. A call meter signal receiver as set forth in claim 9, wherein the receiving frequency of said frequency tuned amplifiers are stabilized by a phase locked loop circuit, and said receiving frequency is tunable to the frequency of call meter signals sent from central office (CO) by varying externally attached circuit of a capacitor and resistors.